Radiation therapy relies on devising a treatment plan, which includes the arrangement of therapeutic radiation beams, patient positioning relative to the beams, beam energies, apertures, and doses, and other factors. Typically, the treatment plan is based on a three-dimensional (3D) computed tomography (CT) dataset acquired prior to the first treatment session. Although CT scans provide a good physical map of electron density within the patient, they also have limitations. For example, CT scans are devoid of functional information about tumors and provide poor soft-tissue contrast for some organs. To circumvent these limitations, other secondary images may also be acquired, using modalities such as positron-emission tomography (PET), magnetic resonance imaging (MRI) or ultrasound. PET, for example, gives functional information about tumor metabolism, and MRI and ultrasound give superior soft-tissue contrast for some organs.
After the treatment plan is developed, the patient is positioned in the treatment room using external skin markings and radiation is delivered according to the treatment plan. This is typically repeated for a number of sessions, for example, once a day for 30 sessions. During this time period, however, a patient's internal anatomy may change. For example, it is known that the prostate can change positions significantly depending on rectal and bladder filling. In an attempt to provide more accurate delivery of radiation therapy, image-guided radiotherapy (IGRT) has become more common. Using IGRT, an image is acquired prior to each session and used to correct the treatment plan for anatomical changes. In principle, a completely new treatment plan can be generated prior to each treatment session—a technique known as adaptive radiotherapy (ART). Although effective, ART is not generally undertaken because re-planning is time-consuming and must be validated and approved for each session. Instead, current clinical practice commonly corrects for physiological changes by shifting the patient (using the treatment couch, for example) in order to best align the target anatomy to the planned location. This is accomplished by comparing the target structure position to its position on a reference image acquired during planning.
One common technique for implementing IGRT is portal imaging, i.e., using the treatment beam to acquire images with either film or a two-dimensional (2D) electronic portal imaging detector (EPID). Due to the high energy (megavolt range) of the treatment beam, image quality is generally inferior to diagnostic (kilovolt range) x-ray images, and provides little or no soft-tissue contrast. Portal images can be effective, however, for localization of bony anatomy, air pockets, and imaging skin surface. One advantage to using the IGRT approach is that the information is inherently acquired by and related to the treatment beam. To ensure the treatment position is correct relative to bony anatomy, the portal images are compared to digitally reconstructed radiographs (DRRs), which are the reconstruction of a 2D projection radiograph from a given beam direction, calculated from the planning CT dataset. EPIDs have been developed not only for electronic record-keeping, but also to make the acquisition more rapid, and to allow online corrections to patient position prior to each treatment fraction. Further, the introduction of flat-panel detectors has improved image quality of EPIDs such that it is comparable to conventional film-based imaging.
Software has been developed to enable rapid displacement calculations using localization images. Typically, 2D structures, called overlays, are extracted from CT contours and displayed on the DRRs on a console. EPID images are acquired from (typically) two angles, such as anterior-posterior and lateral, and the DRR overlays are shown superimposed on the port films. The operator then moves the overlays such that they fit the anatomy as seen on the EPIDs, and the amount of shift is calculated. This allows the therapist to displace the couch to compensate for any discrepancies.
Since portal images do not show soft-tissue contrast, one practice facilitating IGRT is to implant, in the treated organ (e.g., prostate), gold seeds that may be identified on the portal images. By comparing these positions to those on the planning CT, shifts can be executed to correct for organ motion. The use of seeds is invasive, however, and the resulting images do not give a complete picture of the organ and surrounding anatomy.
Another approach includes placing a conventional or cone-beam CT scanner in the treatment room. These scanners generate 3D images, and can either be of diagnostic quality or can use the high-energy treatment beam to produce the 3D images (referred to as megavoltage CT). These images provide a good geometric image of the patient, are similar in nature to the planning CT, and have some soft-tissue contrast which can be used to perform IGRT. For some sites such as prostate, however, fiducial marker seeds are typically used because the soft-tissue contrast is still unacceptable.
Ultrasound has also been used for IGRT, as it provides good soft-tissue contrast. Two or more ultrasound images are referenced to a 3D coordinate system, and are either used individually or are reconstructed to a full 3D image dataset. Patient displacements can then be determined from the ultrasound images. Although they can provide excellent soft-tissue contrast for organs such as the prostate, uterus or breast tumor cavities and do not require fiducial markers, ultrasound images do not give bony anatomy, or a complete anatomical image of the patient.
There has been research and development into the use of in-treatment-room MRI and PET for IGRT, but technical hurdles remain before this technology becomes commercially available.